1. Field of the Invention
The present invention relates to the oxidation of sodium sulfide in Kraft cooking liquors. More specifically, the present invention relates to a method of selectively oxidizing sodium sulfide to sodium polysulfide in Kraft cooking liquors, where a clarified white liquor is used.
2. Brief Description of the Related Art
In the conventional Kraft cooking process, two chemicals, namely sodium hydroxide and sodium sulfide, are used to delignify the wood chips. During the course of the reaction, part of the undesired fraction of wood, lignin, is solubilized and removed. However, cellulose and hemicelluloses, which are desirable components, are also attacked. Hence, one of the goals sought during cooking is to protect this fraction in order to achieve a better process yield.
Theoretically, it should be possible to fully retain cellulose and hemicelluloses. The weight contribution of these components varies with each wood species but is usually around 70%. However, in an industrial process, the amount retained is more in the order of 45-50%. Typically, 80% of the lignin, 50% of the hemicelluloses and 10% of the cellulose are removed. The hemicelluloses are easily attacked since they are low molecular weight sugars that are more accessible than crystalline cellulose. The mechanism by which they are removed is called alkaline peeling and occurs at the reducing end group of the polymeric chain.
It is well known that in order to increase the carbohydrate yield in the Kraft cooking process, polysulfides can be introduced in the digester. This prevents the degradation of the polysaccharides and increases the yield for a given lignin content. This concept was first discussed by Haegglund in 1946 (Svensk Papperstidn. 49(9):191, 1946).
Polysulfides can be generated by various, different means. In one approach, polysulfides are formed by adding elemental sulfur to the white liquor. However, adding elemental sulfur leads to imbalances in the sulfur balance of the chemical recovery cycle. The build up of sulfur will eventually be released to the atmosphere as a sulfur gas emission. For this reason, this approach has very limited industrial interest.
A second approach consists of chemically oxidizing the sodium sulfide present in the white liquor to sodium polysulfides. The resulting polysulfide liquor is known in the art as orange liquor. This method involves several chemical species, but in general, assuming a polysulfide chain length of n=2, the chemical reactions can be written as follows:2HS−+2O2S2S−2+2OH−+2H2O  (1)2S2S−2+4O2+2OH−3S2O3+H2O  (2)2HS−+2O2S2O3−2+H2O  (3)2HS−+3O22SO3−2+2H+  (4)2SO3−2+O22SO4−2  (5)
One goal sought during the oxidizing is to maximize the formation of polysulfides and minimize the formation of dead load and more specifically thiosulfate. This is measured by selectivity, a term known in the art which corresponds to the amount of polysulfides formed/amount of converted sulfide on a sulfur basis.
Several variations of this oxidative method have been published. In U.S. Pat. No. 3,470,061, Barker discloses a method using inorganic manganese oxides as the oxidant. In this respect, the chemical equation involving polysulfides can be written as:MnO2+2Na2S+H2OMnO+Na2S2+2NaOH  (6)
Once reduced, the catalyst is reoxidized with air or oxygen after separation from the white liquor according to:MnO+½O2MnO2  (7)
This oxidation is performed in an external recycle loop after the catalyst has been separated and dried. However, said process has several drawbacks. In particular, the described process requires a long retention time for reaction, e.g., up to 20 minutes. As well, the described process does not teach the importance of location wherein the white liquor is used for polysulfide preparation. For example, the white liquor prior to the clarification step contains a large amount of lime mud. Using white liquor containing sodium sulfide prior to the clarifier can result in problems due to the lime mud.
In U.S. Pat. No. 3,860,479, Barker discloses a method in which the manganese dioxide catalyst is regenerated in situ without the need of an external recycle loop. This process still has many of the drawbacks of U.S. Pat. No. 3,470,061, as it still requires large retention times.
In U.S. Pat. No. 4,024,229, Smith discloses a method to generate polysulfides by chemical oxidation using particulate carbon, coated with a PTFE, as the catalyst. The method is said to reduce the production of thiosulfate. However, the catalyst bed has to be regenerated due to deactivation of the catalyst by particles of calcium carbonate.
In U.S. Pat. No. 4,855,123, Suzuki et al. disclose a method similar to that of U.S. Pat. No. 4,024,229. However, in this case, the catalyst is activated carbon. This invention offers the same drawbacks as the previous disclosure.
In U.S. Pat. No. 5,082,526, Dorris discloses a method to produce polysulfides in the presence of lime mud. The disadvantages of this method is that it requires long oxidation times which leads to a lower selectivity because of overoxidation and thermal degradation of the polysulfide. Another problem is that all the white liquor with its lime mud must be sent to the oxidation process, which increases oxygen usage and equipment cost.
In U.S. Pat. No. 5,624,545, Landfors et al. disclose a method to produce polysulfides by electrolysis of the white liquor. Said method has the drawback of having high capital and energy cost.
In WO patent 97/42372, Yant et al. disclose a method to produce polysulfides from white liquor. In this process an inorganic metal is used as a catalyst, similarly to U.S. Pat. No. 3,860,479. The catalyst is then separated by centrifugal action and reintroduced with an oxygen-containing gas into a specially designed reactor. However, said process has the drawbacks of requiring a large footprint, high capital costs and large amounts of catalyst.
Therefore, there are many different processes available to produce polysulfides from white liquors to thereby increase the yield in Kraft cooking. However, the processes are generally either complicated, or less than cost effective. It is therefore an object of the present invention to provide a simple and efficient method for producing polysulfides without the drawbacks associated with the prior art methods.
Another object is to provide an improved, cost effective/efficient process for the oxidation of sodium sulfide to sodium polysulfide in Kraft cooking liquors.
Yet another object is to provide such a process which increases the production of sodium polysulfides and which decreases the amount of sodium thiosulfate dead-load.
These and other objects of the present invention will become apparent upon a review of the following specification, the figures of the drawing, and the claims appended thereto.